HONG KONG (Reuters) - Li Ka-shing's CK Asset Holdings Ltd (>> CK Asset Holdings Ltd) is selling out of a central business district office tower for HK$40.2 billion (3.88 billion pounds), marking Hong Kong's biggest real estate deal.

CK Asset, formerly Cheung Kong Property Holdings Ltd, said on Wednesday it would sell its 75 percent interest in the 73-storey office tower, The Center, to C.H.M.T. Peaceful Development Asia Property Limited.

Chinese firms have been aggressively expanding in Hong Kong property, one of the world's most expensive real estate markets, buying 29 percent of land sold in 2015 and 2016.

Several such buyers had expressed a strong interest in acquiring the CK Asset office tower, although Postal Savings Bank Of China Co Ltd (>> POSTAL SAVINGS BANK of CHINA Co Ltd) made clear a year ago that it did not plan to buy the property.

CK Asset said in a filing that it expects to record a gain of about HK$14.5 billion from the sale, with the proceeds to be used for general working capital purposes.

Shares in the company controlled by the Hong Kong businessman have risen more than 36 percent this year, outpacing a 30 percent gain in the broader market <.HSI>.

The deal follow Mainland developer LVGEM (China) Real Estate Investment Co Ltd (>> LVGEM (China) Real Estate Inve Co Ltd) saying in mid-October that it would buy an office tower from Wharf (Holdings) Ltd (>> Wharf Holdings Ltd) for HK$9 billion to strengthen its presence in Hong Kong property.

(Reporting by Donny Kwok and Twinnie Siu; Editing by Alexander Smith)